This invention relates to test tube cap assemblies designed to seal and support the upper end of a test tube during centrifugation. More specifically, this invention is directed to an improved test tube cap assembly sealing and support arrangement designed for use with rotos which experience considerable hydraulic pressure at the interface between the test tube and the cap assembly.
The test tubes normally utilized in high speed centrifuge rotors have a thin wall construction and, therefore, are subjected to deformation as a result of centrifugation forces when there is not adequate liquid support within the test tube.
In the case of centrifuge rotors having pivotal test tube cavities, the longitudinal axis of each tube assumes an angle that is substantially perpendicular to the spin axis of the rotor. The rotor tube cavity is closed by means of a cap and seal, but the functioning of neither of these components is affected by the tube or its contents. In effect, the upper end of the tube is unconstrained by a tube cap. The tube, however, must contain sufficient liquid to support the interior of the upper end of the tube to prevent it from buckling under its own centrifugal weight. The liquid level in the tube has no bearing on the integrity of the seal in these pivotal test tube cavity rotors.
However, in the case of other rotors the longitudinal axis of the test tube cavities is at an angle less than 90.degree. with the rotational axis of the rotor. For such rotors it is necessary to provide a tube cap which not only seats, but also supports the upper end of the tube against the action of centrifugal force. Generally, the contained liquid does not completely fill the test tube and/or the tube stretches to fill its rotor cavity, resulting in the formation of an air pocket at the centripetal side of the test tube. The centrifugal forces acting on the centripetal side of the test tube will tend to pull the upper portion of the test tube out of the tube cap at the centripetal side of the rotor cavity, causing the seal to fail.
During centrifugation the test tube cap and tube assembly interface will experience some hydraulic pressure from the fluid therein in conjunction with the centrifugal forces that also tend to pull the upper end of the tube out of the tube cap. Consequently, test tube cap arrangements had to be devised to resist the forces which tend to cause leakage and/or tube collapse. Examples of such prior art approaches to avoid such problems in a fixed angle rotor during centrifugation are disclosed in Galasso et al. U.S. Pat. No. 3,447,712, Marks U.S. Pat. No. 3,459,369 and Wright et al. U.S. Pat. No. 3,938,735.
When the angle of the longitudinal axis of the test tube with respect to the rotational axis of the rotor decreases toward zero, the hydraulic forces exerted on a tube cap assembly become significantly greater, requiring greater sealing or counter forces to prevent leakage of the fluid sample and to prevent tendency of the tube to pull out of the centripetal side of the cap. One approach to counter the higher hydraulic forces is the use of a plug threadedly mounted within the rotor test tube cavity over the test tube cap assembly. A patent application entitled Dual Seal Arrangement for a Centrifuge Rotor Tube Cavity, Ser. No. 715,117 filed Aug. 17, 1976 by Roger Anderson and John Edwards uses a secondary seal between such a plug and the tube cap to provide greater assurance that any seepage from the test tube itself would be contained within the rotor and not escape the rotor during centrifugation. However, there is still the concern and desire to always contain the fluid sample completely within the test tube and avoid any leakage out of the test tube. Otherwise, the integrity of the centrifugation run would be destroyed.
In some rotors the test tubes have been vertically oriented, so that the longitudinal axis of the test tube is essentially parallel to the rotational axis of the rotor. However, the hydraulic forces exerted by the fluid sample on a test tube cap assembly are greatly increased, accentuating potential leakage problems of the fluid sample. Hence, there is a need for an uncomplicated, but secure seal in a tube cap assembly for vertical or nearly vertical tube rotors.